1. Field of the Invention
The present invention relates to an optical receiver that receives an optical signal of broad-range illumination and converts the optical signal to an electrical signal, and also relates to a data communication apparatus comprising same.
2. Description of the Related Art
A data communication apparatus that performs data communications by using an infrared optical signal between computers or between a computer and peripheral equipment conforms to the IrDA (Infrared Data Association) standard. Such a data communication apparatus (see, for example, Japanese Patent Application No. 2001-230726) uses an optical receiver that receives an optical signal and converts the optical signal into an electrical signal, and a conventional example of the optical receiver is shown in FIG. 4. An optical receiver 101 is constituted by a photodiode 112, which receives an optical signal and converts the optical signal into a current IPD; an impedance element 110 constituted by a resistor, for example, that detects the current IPD and converts the current IPD into a voltage signal; a clamp element 111, which clamps the voltage signal (limits the amplitude thereof); a capacitor 121, which removes the DC voltage of the voltage signal converted by the impedance element 110; an inverting amplifier 122, which inverts and amplifies the voltage signal from which the DC voltage has been removed; and a comparator 127, which outputs, via an output terminal OUT, a signal (electrical signal) that results from a comparison of the output signal of the inverting amplifier 122 with an output reference voltage of an output reference power supply 128. A signal processing device (not shown), which processes an electrical signal corresponding to the received optical signal, is connected to the output terminal OUT.
Further, according to the IrDA standard, the illumination of the optical signal to be received by an optical receiver that is suited to a variety of environments in which data communications are performed is broad-range illumination (from 10 μW/cm2 to 500 mW/cm2, for example). FIG. 5 shows the current IPD that flows to the photodiode 112 (the current converted by the photodiode 112), the voltage signal from the impedance element 110, that is, the voltage signal VA at node A in FIG. 4, and the signal of the output terminal OUT, when pulses of optical signal of different illumination are received Case (1) of FIG. 5 represents a case where the optical signal is within the illumination range of the IrDA standard but the illumination is low; Case (2) of FIG. 5 represents a case where the illumination is comparatively high and the clamp element 111 has started operating; and Case (3) of FIG. 5 represents a case where the illumination is higher still and the clamp element 111 fully operates. Thus, the optical receiver 101 is made to correspond with an optical signal of broad-range illumination by limiting the voltage by use of the clamp element 111 in the case where the illumination is high. In FIG. 5, ICLAMP is the current value at which the clamp element 111 operates. The voltage VthA is rendered by converting the output reference voltage that is inputted to the comparator 127. When the voltage signal VA at node A is lower than VthA, a high level is outputted to the output terminal OUT. When the voltage signal VA at node A is higher than VthA, a low level is outputted to the output terminal OUT.
Thus, the optical receiver 101 is able to output a pulse-like electrical signal, which is a logical signal, from the output terminal OUT in correspondence with an optical signal of broad-range illumination. However, when the illumination of the optical signal is high, the photodiode 112 enters a saturated state, and, hence, extra time is required to restore the original state after the pulse end of the optical signal by eliminating the electron/positive-hole pairs that have accumulated in the photodiode 112, and the current IPD continues to flow over this interval. Therefore, as shown by Cases (1), (2), and (3) of FIG. 5, the pulse width (tPW1, tPW2, tPW3) of the output terminal OUT expands as the illumination of the optical signal increases.
Meanwhile, IrDA-standard data communications include high-speed data communications with a communication speed of 4-Mbps, and corresponding devices have also been produced. When the pulse width fluctuates in such high-speed data communications, the load on the signal processing device, to which the pulse from the output terminal OUT is inputted and processed, is large and, hence, in extreme cases, the occurrence of a signal-processing malfunction is a matter of concern. A higher-speed 16-Mbps data communication standard has also been established. It is assumed that, in this case, the load on the signal processing device is even greater and that malfunctions may occur more often.